A survey of high-frequency SiO masers in supergiants and AGB stars

We report the results of a survey of high-frequency SiO maser emission from a sample of 34 late-type supergiant and AGE stars. Four new sources were d etected, including the first detection of 301-GHz emission from an S-type s tar. Variability of the maser emission with stellar phase is discussed in d etail. We find that high-frequency SiO maser emission appears to be excepti onally weak or absent in Mira variables over an optical phase range of appr oximately 0.4 to 0.7. No significant correlation is found between maser pho ton luminosity and spectral type for non-supergiant types. Centroid velocit ies of spectra appear to be redder than the stellar velocity over a phase r ange that is wider than the band where emission is weak on average. However , the conclusion that only the blue features appearing in spectra after an optical phase of 0.7-0.8 correspond to new masing objects should be viewed with caution.